Most conventional wheel structure allows only for braking, and only a small portion used for medical purposes may allow wheel structure move toward a straight direction. However, although both the direction positioning component and the braking component of these specialized carts are usually installed to the wheel structure and are operated by foot pedals, the direction positioning component however needs to be controlled separately from the braking component, which is rather inconvenient. A different type of wheel structure that is commercially available have an improved controlling structure which allows the wheel structure to rotate freely, to move in a preset straight direction, and to be stopped by a braking operation, and thus the transporting devices equipped with this type of improved wheel structure possesses desirable features for practical purposes. However, this improved wheel structure is braked by the fraction force generated when the braking pads contact the surfaces of a wheel bodies, and the wearing caused by the braking operation, especially as the braking operation is frequently needed, will reduce the thickness of the braking pads as well as the wheel bodies and then increase the gap between the functioning surfaces of the braking pads and the wheel bodies. As a result, the efficiency of the braking operation will be reduced and transporting devices equipped with this type of wheel structure may not be effectively stopped as the braking components may not function properly after being used for a certain period of time. To solve this problem, the wheel structure has to be replaced for normal braking performance, which is inconvenient, time- and labor-consuming, and may incur extra monetary costs. Therefore, there remains a need for a new and improved wheel structure to overcome the problems stated above.